Your friend needs to do some research with the AFCO because military police work is quite different to the civilian police force. There is no 'beat' work equivalent in the military police, and their powers to arrest, detain, investigate are quite different. The RMP website explains this in some detail.
I don't think there is much difference between the type of work done in the RMP or the RAFP. Though I presume the RMP is a much larger organisation. I think all Police are promoted to acting Corporal on qualification, but other than that, it doesn't have the reputation, at least in the RAF, of being a fast promotion trade. I presume promotion is very closely aligned to further qualifications.
As to pay, it will be standard for the rank and is available on the web or from the AFCO- try googling the Armed Forces Pay Review Body latest report.